Cupel dust

from The Collaborative International Dictionary of English v.0.48

Cupel \Cu"pel\ (k[=u]"p[e^]l), n. [LL. cupella cup (cf. L.
   cupella, small cask, dim. of cupa) : cf. F. coupelle. See
   {Cup}, and cf. {Coblet}.]
   A shallow porous cup, used in refining precious metals,
   commonly made of bone ashes (phosphate of lime). [Written
   also {coppel}.]
   [1913 Webster]

   {Cupel dust}, powder used in purifying metals.
      [1913 Webster]